1. Field of the Invention
The present invention relates to a device and a method for determining a filling difference in cylinders of an internal combustion engine having at least two cylinders.
2. Description of the Related Art
The air/fuel ratio in Otto-cycle engines is usually set in such a way that the average of the lambda values of all cylinders (so-called “total lambda”) λ is equal to 1.0. This makes possible low-emissions operation, since catalytic converters exhibit their best effectiveness with stoichiometric combustion.
As a result of metering tolerances and air/filling differences between individual cylinders, e.g. as a result of system tolerances, the lambda values in the individual cylinders of an internal combustion engine can deviate from one another despite identical control application. The total lambda measured in the exhaust, which total is made up of the contributions of the respective individual cylinders, can therefore assume the target value λ=1.0 even though the lambda values of the individual cylinders fluctuate around that average. A corresponding deviation of individual cylinders from the average is also referred to as a “cylinder inhomogeneity.”
A cylinder inhomogeneity has a number of disadvantages. A shift in the individual-cylinder lambda values firstly results directly in an increase in fuel consumption. If a specific threshold is exceeded, emissions become worse. The so-called “stringiness” of the exhaust gas, i.e. the formation of flow strands in the exhaust mass flow as a result of, for example, filling differences, additionally plays a role here.
At a constant air/fuel ratio the power parameters of an engine, more precisely of a cylinder, are proportional to the mass of air or mixture delivered to the cylinder, i.e. to the volumetric efficiency. The indices that serve to define the volumetric efficiency are, as generally known, the delivery ratio and the charging efficiency. If the volumetric efficiency values of the cylinders deviate from one another, their torque contributions—i.e. the respective cylinders' shares of the total torque—also differ. This causes irregularities in engine speed.
When “power output parameters” or more generally “power output” is discussed in the context of this invention, this term is not to be understood as being limited to a power output in the sense of a physical variable. The terms instead also encompass, for example, a torque as well as an indicated and/or effective mean pressure of a cylinder. Such indices are linked via conversions to one another and to the power output of a cylinder, and define it.
Reliable methods for recognizing filling differences are not yet available for Otto-cycle engines, and for that reason a corresponding need exists.